I already mentioned it before. ONLY Tenshi can use Guard skills because they were made with HER experiences/emotions/whatever Tenshi's own ingredients are. And well, Yuri has running low on time, she may have thought that Harmonics was the most dangerous one so far (therefore sabotaging it), I mean making a copy would mean 2x the trouble.

True on the first point hasn't been shown what it takes to use them.

I have to disagree on the second part, her distortion, and delay are the most dangerous skills really as they make her extremely difficult if not impossible to beat in range/melee combat respectively. Thus disabling one of them would make a more logically choice, since without distortion even if Tenshi made 5 copies it wouldn't matter much when the SSS could gun them on down[or at least stop her movement by shooting her legs/arms.]